fix(P1): clean disk before release + fallback to existing binary

The release job fails in 5 seconds — likely due to disk full (CoreCI's
SQLite logging fills the disk during go test -race). Fix:
1. Clean up coverage.out and Go build cache before the release job
2. If go build fails (disk full), fall back to the existing bin/orca
   from the build job (which succeeded)
3. Add more error handling for tar/sha256sum

---ci---
project: orca
phase: 1
milestone: v0.16
status: execute
---/ci---

scripts/ci-run.sh

@@ -100,8 +100,6 @@ case "$JOB" in
   # ── release ────────────────────────────────────────────────────────
   release)
     VERSION="${CI_COMMIT_BRANCH:-dev}"
-    GIT_COMMIT="${CI_COMMIT_SHA:-unknown}"
-    BUILD_TIME="$(date -u +%Y-%m-%dT%H:%M:%SZ)"
     TARBALL="orca-${VERSION}-linux-amd64.tar.gz"
 
     if [ -z "${GITEA_TOKEN:-${CI_GITEA_TOKEN:-}}" ]; then
@@ -109,19 +107,36 @@ case "$JOB" in
     fi
     GITEA_TOKEN="${GITEA_TOKEN:-${CI_GITEA_TOKEN:-}}"
 
-    info "building release binary ${VERSION}..."
-    info "GITEA_TOKEN length: ${#GITEA_TOKEN}"
-    info "CI_COMMIT_BRANCH=${CI_COMMIT_BRANCH:-unset}"
-    info "CI_COMMIT_SHA=${CI_COMMIT_SHA:-unset}"
+    info "release ${VERSION} — GITEA_TOKEN length: ${#GITEA_TOKEN}"
+
+    # Clean up disk space — CoreCI's SQLite logging can fill the disk.
+    # Remove coverage.out and Go build cache to free space.
+    rm -f coverage.out 2>/dev/null || true
+    go clean -cache 2>/dev/null || true
+
+    # Rebuild the binary (the build job's binary may not persist if the
+    # executor uses separate workdirs, or the disk was full).
+    info "building release binary..."
+    GIT_COMMIT="${CI_COMMIT_SHA:-unknown}"
+    BUILD_TIME="$(date -u +%Y-%m-%dT%H:%M:%SZ)"
     LDFLAGS="-s -w \
       -X git.cloudinit.dev/coreci/orca/internal/cli.version=${VERSION} \
       -X git.cloudinit.dev/coreci/orca/internal/cli.gitCommit=${GIT_COMMIT} \
       -X git.cloudinit.dev/coreci/orca/internal/cli.buildTime=${BUILD_TIME}"
     mkdir -p bin
-    go build -trimpath -ldflags="${LDFLAGS}" -o bin/orca ./cmd/orca 2>&1 || err "go build failed"
-    tar -czf "${TARBALL}" -C bin orca
-    sha256sum "${TARBALL}" > SHA256SUMS
-    info "built ${TARBALL} ($(wc -c < "${TARBALL}") bytes)"
+    go build -trimpath -ldflags="${LDFLAGS}" -o bin/orca ./cmd/orca 2>&1
+    BUILD_EXIT=$?
+    if [ ${BUILD_EXIT} -ne 0 ]; then
+      info "go build failed (exit ${BUILD_EXIT}) — trying with existing bin/orca from build job"
+      if [ -x bin/orca ]; then
+        info "found existing bin/orca from build job"
+      else
+        err "go build failed and no existing binary found"
+      fi
+    fi
+    tar -czf "${TARBALL}" -C bin orca 2>&1 || err "tar failed"
+    sha256sum "${TARBALL}" > SHA256SUMS 2>&1 || err "sha256sum failed"
+    info "built ${TARBALL}"
 
     # Create or update the Gitea release directly via the API.
     # The CIAgent ship workflow may have already created the release